Performance of cesium telluride photocathodes as an electron source for the Los Alamos Advanced FEL

The Los Alamos Advanced FEL was successfully operated with a Cs2Te photocathode driven by a frequency quadrupled Nd:YLF laser as the electron source. Lasing was achieved at 5–6 μm. Cs2Te photocathodes with quantum efficiencies of 15–18% at 251 nm were fabricated in an ultrahigh-vacuum chamber and transferred under high vacuum to the FEL. 263 nm light from the drive laser was focused to an 8 mm spot on the center of the photocathode. We estimated the operational lifetime of Cs2Te photocathodes to be at least 20 times that for K2CsSb photocathodes. We have found that the dark current, emittance of the extracted beam, and amount of charge extracted is sufficient for FEL applications.